Photosensitive element having photoconductive layers



June 22, 1965 R. M. WILMOTTE 3,191,045

PHOTOSENSITIVE ELEMENT HAVING PHOTOCONDUCTIVE LAYERS Original Filed Dec.11, 1961 4 Sheets-Sheet l F l G. I

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INVENTOR. ROBERT COLMAN BYyMgi ATTORNEY June 22, R M w o PHOTOSENSITIVEELEMENT HAVING PHOIOCONDUCTIVE LAYERS Original Filed Dec. 11, 1961 4Sheets-Sheet 2 F l G. 2

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6 ROBERT COLMAN z 0 WHQV ATTORNEY June 1965 R. M. WILMOTTE 3191Q45PHOTOSENSITIVE ELEMENT HAVING PHOTOCONDUCTIVE LAYERS Original Filed Dec.11, 1961 4 sheets-sheet 4 FIG. 7

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INVENTOR. ROBERT COLMAN ATTORNEY United States Patent Ofiice 3,191,045Patented June 22, 1965 3,191,045 PHOTOSENSITIVE ELEMENT HAVINGPHOTOCONDUCTIVE LAYERS Robert Colman, New York, N.Y., assignor, by mesneassignments, to Ciairex Corporation, New York, N.Y., a corporation ofNew York Original applications Dec. 11,1961, Ser. No. 158,344, newPatent No. 3,142,586, dated July 28, 1964, and llbec. 11, 1961, er. No.158,368. Divided and this application Sept. 11, 1952, Ser. No. 230,021

10 Claims. (Cl. 250-211) The application is a division of copendingapplication Serial No. 158,334, filed December 11, 1961, now Patent No.3,142,586, issued July 28, 1964, and of copending application Serial No.158,368, filed December 11, 1961, now abandoned.

This invention is directed to improvements in photoconductive cells inwhich a change in electrical resistance occurs as a result of a changein illumination thereof, and to improvements in the manufacture of suchcells.

A serious limitation in the manufacture of photoconductive devicesresults from the limited wavelength sensitivity of particularphotoconductive materials employed in such devices. For example, cadmiumsulfide, one of the most commonly used photoconductive materials, has aresponse bandwidth of only about 40 millimicrons at a peak responseWavelength of about 5200- 5300 A. In the case of photoconductive cellsused in photography or as detectors of radiation, or in otherapplications, it is frequently desirable to obtain a broad bandwidthover a pre-selected, peak response spectral region.

With the conventional photocells, however, peak response bandwidth andwavelength characteristics are determined by the photoconductivematerials employed, and accordingly, the job frequently has to betailored to the photocell, rather than the photocell being tailored tothe job. Obviously, such a situation leaves much to be desired.

It is an object of the present invention to provide photoconductiveelements having predetermined, board bandwidth, peak spectral responsecharacteristics, the characteristics being independent of the activityof the cell and being at least in part, a function of the physicaldimensions of the cell.

Another object of the present invention'is to provide photoconductiveelements of the type described having peak spectral response andbandwidth characteristics which are different from the peak spectralresponse and bandwidth characteristics of the photoconductive materialsfrom which the elements are made.

Another object of the present invention is to provide photocellsimproved photosensitivity.

A further object of the present invention is to provide methods formanufacturing photosensitive elements having any of the aforementionedpredetermined properties.

Other and further objects and advantages of the present invention willbecome apparent from the following description taken together with thedrawings, wherein:

FIGURES 1 to 5 are charts useful in explaining the invention;

FIGURE 6 is a schematic flow sheet illustrating the steps in themanufacture of photosensitive elements according to the presentinvention;

FIGURE 7 is an isometric view of a photosensitive element made accordingto the teachings of the present invention;

FIGURE 8 is a top view of the photosensitive element shown in FIGURE 7;

FIGURE 9 is a cross section of another embodiment of the photoconductiveelement;

FIGURE 10 is a diagrammatic illustration of an apparatus suitable foruse in preparing an embodiment of the present invention.

The photosensitive elements of the present invention comprise aplurality of layers or strata of a photoconductive material, thesurfaces of each layer being in immediate, intimate and overall contactwith each other, and each layer being individually deposited andindividually light sensitized as taught herein prior to deposition ofthe next succeeding layer.

Each of the multiple layers making up the elements disclosed herein isin the form of an extremely thin plate or platelet, the surface area ofthe plate or platelet being large in comparison with the thickness. Ingeneral, each of the layers will have a uniform thickness of betweenabout 1.5 and 4.5 microns.

As will be described more fully hereinbelow, with reference to FIGURE 6,the layers or strata will be co-terminus or substantially co-terminus insurface area, so that the layers will be in overall or substantiallyoverall planar contact with each other.

The photosensitive elements may be prepared by depositing, successively,a plurality of layers of a single polycrystalline photoconductivematerial. Following deposition of each layer or stratum, and prior todeposition of the next succeeding layer, each deposited layer issensitized by heating at temperatures above at least C., usually betweenabout 300 and 900 C., and preferably between 500 and 700 C., and thencooled to ambient or room temperature. The temperature during heatingmay, if desired, approximate sintering temperature. The atmosphereduring heating and cooling may be an inert gas, such as nitrogen or anyof the so-called noble gases, e.g., argon, neon, and so forth. Ordinaryatmospheric air is, however, satisfactory. Also, the atmosphere maycontain activating or modifying agents, as will be made more clearhereinbelow. For best results the heating should be carried out in thepresence of a halogen, e.g., chlorine, iodine, and bromine, and mixturesof the foregoing, preferably chlorine. The layers may be deposited byany of the well known techniques, such as by evaporation, spraying,floating or chemical deposition.

Heating the layers in the presence of a halogen has been found tophotosensitize the host photoconductive ma terials in each layer. Forthe results herein described to be obtained, it is necessary tophotosensitize each layer prior to deposition of the next succeedinglayer.

Different, pro-selected, peak bandwith and wavelength spectral responsecharacteristics may be obtained by varying the thickness and the numberof the layers or strata described. Depending upon the response desired,and the particular photoconductive material used, the thickness of eachlayer may vary between about 1.5 and 4.5 microns, and is preferablybetween about 2 and 4 microns. At the lower limit, the number of layersemployed will be at least 2. The number of layers that can be used inexcess of 2 is theoretically infinite, each additional layer giving arefinement in the peak response bandwidth and the peak responsewavelength. As a practical matter, however, the number of layers willvery rarely be in excess of 10, and will ordinarily be less than 5, orbetween 2 and 5, including 2 and 5.

With the photoconductive elements described herein, the peak responsebandwidth and the peak response wavelength are both different from thosecharacteristics of the photosensitive materials making up the layers.

The photoconductive material used to prepare each layer may be a singlepolycrystalline photoconductive material, or a mixture ofpolycrystalline materials. Al though any of the well knownphotoconductive materials may be employed, best results are obtainedwith the sulfides, selenides, and tellurides of cadmium, zinc andmercury. Of these materials, the cadmium saltsare preferred.

For each of the layers described, one of the photoconductive materialsdescribed herein, or a mixture thereof, will be selected. This will bereferred to as th host material for the reason that this basic or hostphotoconductive material may be altered by activators to increase thelight sensitivity of the elements, or by modifying agents to shift evenfurther the peak response wavelength, or to broaden the peak responsebandwidth, as will be made clear hereinafter.

. The host photoconductive material need not be in compound form whendeposited as a layer. Thus, when the photoconductive material is cadmiumbased, the layer deposited may be a mixture of elemental cadmium withelement-a1 sulfur, tellurium, or selenium, Similar elemental mixturesmay be used when the photoconductive material is zinc or mercury based.

Regardless of the type or combination of photoconductive materials used,it should be noted that the multi-layer elements described herein havepeak bandwidth and wavelength characteristics difier-ent from the peakbandwidth and Wave length characteristics of the photoconductivematerials along. I

With some of the photosensitive host materials, the use of activatorsmay be desirable to increase the activity, or more accurately,sensitivity of the elements. -Such activators are well understood in the.art, and include small amounts, e.g., 10 ppm. to 1000 p.p.m.',preferably about 50 ppm. to 500 ppm, ofcopper, silver, gold, aluminum,silicon and halogens, including mixtures of the foregoing.

The activators may be incorporated into the host polycrystallinematerial or materials either at the time the individual layer isdeposited, or during the heating step. In either event, the activatorwill be present in the layer during heating. Regardless of whether otheractivators are present, as had already been brought out, the heatingwill take place in the presence of a halogen.

In each of FIGURES 1 and 2, characteristic curves are illustrated forvarious photocells, each of which contains a photoconductive element.

The host or primary photoconductive material for each and every curve inFIGURES 1 and 2 is polycrystalline.

cadmium sulfide.

The method of preparing the photocells whose characteristics areindicated by the curves of FIGURES l and 2 will be clear from thefollowing examples.

Example 1 For comparison purposes, uni-layered photosensitive elementsof varying thicknesses were prepared as follows:

(1) very finely ground powder ;of spectrographically pure then flowed ona refractory support with the crystals still in suspension. The granuleswere allowed to settle on the support uniformly. The excess liquid wasthen allowed to drain off and the granules allowed to dry at roomtemperature; (4) the coated support was then heated in an oven in thepresence'of chlorine for 2 hours at 500 to 700 C.,' (5) the heatedsupport was then allowed to cool to ambient room temperature.

Using this procedure, three photoconducting elements, A, B and C, havinga thickness of 2.8 microns, 5.6 microns and 11.2 microns, respectively,were prepared.

Terminals were attached to the top of each cell as shown in FIGURE 7,and spectral response characteristics were measured.

To obtain the curves shown in FIGURES 1 and 2, each photocell orphotosensitive element was illuminated by a series of light beams atdifferent wavelengths and constant incident radiant power while aconstant voltage (60 volts D.C.) was applied to the terminals, and theresulting DC. currents passing through the photocells were meas ured..The maximum current reading for each element was designated as 100percent relative response, and all other readings were recorded aspercentage responses with respect to the maximum electrical response.

IN FIGURE 1, curves A, B and C illustrate the spectral responsecharacteristics for photosensitive elements of 2.8 microns, 5.6 microns,and 11.2 microns, respectively, prepared .as a single layer or stratumaccording to the procedure of Example 1.

As can be seen from FIGURE 1, increasing the thickness of theuni-layered elements products no change in the peak response, wavelengthor bandwidth. With increasing thicknesses, however, the relativeresponse over the broad endof the spectrum increases, as is indicated bythe wider and higher skirts of the B and C curves to the right of thepeak response wave length, as compared with curve A. 1

Example 2 This example illustrates the method of preparing themulti-layer photoconductive elements of the present in- 1 vention.

A first layer of polycrystalline cadmium sulfide 2. 8 microns thick wasprepared as follows: (1) very finely ground powder ofspectro-graphically pure cadmium sulfide containinglO ppm. copper wasground to particles ranging in average size between about'l and 2microns;

; suspension; The granules were allowed to settle on the supportuniformly. The excess liquid was then allowed to drain off and thegranules allowed to dry at room temperature; (4) the cooled support wasthen heated in an oven in the presence of chlorine for 2 hours at 500 to700 C.; (5) the heated support was then allowed to cool to ambient roomtemperature.

Steps (1) to (5) were repeated exactly to prepare second, third andfourth layers of cadmium sulfide polycrystalline material, each 2.8microns thick.

Using the procedure of Example 1, the spectral response of the elementwas determined following complete formation of the first, second thirdand fourth layers, respectively.

In FIGURE 2, curves F, G, H and I, respectively, indicate the spectralresponse characteristics of the element following deposition and heatingin the presence of halogen of the first, second, third and fourthlayers, respectively.

As can be seen from FIGURE 2, the peak spectral response wavelengthshifts from the short end of the spectrum to the long end with eachadditional layer, the shift being most pronounced between the second andthird layers. Also there is a broadening of the peak response bandwidthas the number of layers increases.

Following deposition and heating of each layer, the element of Example 2has a different spectral response, broader and with a shifting of thepeak response toward the longer wavelength regions of the spectrum, asis evident from FIGURE 2. I

It should also be noted that the overall thickness of the elementscorresponding to curves G and I of FIGURE 2 is identical to the overallthickness of the elements for curves B and C, respectively, of FIGURE 1;and that the elements of curves G and I in FIGURE 2 are identical to theelements of curves B and C of FIGURE 1, with the exception that elementsH and I of FIGURE 2 are made by thernulti-layer technique of Example 2,whereas the elements B and C of FIGURE 1 are made by the unilayertechnique of Example 1.

The advantages of the present invention accordingly will be immediatelyapparent from a comparison of FIG- URES 1 and 2.

It the elements produced according to the multi-layered techniquedisclosed herein are cut into cross sections and examined under amicroscope, following completion thereof, no interface or demarcation ofwhatsoever nature can be found. Additionally, as should already beclear, there is no dielectric or insulation of whatsoever nature betweenthe layers of the elements disclosed herein.

In FIGURE 6, there is illustrated schematically the condition of thephotoconductive element at stages I to V in the process.

In FIGURE 6, s represents the base layer or support which may be anysuitable material, but is preferably a refractory material, such asporcelain or other ceramic, glass, and the like.

In stage I of FIGURE 6, the first layer of photoconductive material, a,has been deposited on the base s, and the layer has been heated in thepresence of halogen and cooled to ambient temperature.

At stage II in FIGURE 6, a second layer, 12, has been superimposed onlayer a, and the physical appearance of the element before heatinglayer 1) is shown.

At stage III in FIGURE 6, the unit has been heated to form the elementa+b, and an additional layer 0, unheated, has been superimposed thereon.

At stage IV in FIGURE 6, the unit has been heated to form the elementa+b+c, and layer d, unheated, has been superimposed thereon.

At stage V in FIGURE 6, the unit has been heated to complete 'themulti-layered photoconductive element a+b+c+d, supported on the base s.

' As will .be clear from FIGURE 6, the layers as deposited down willgenerally be parallel or substantially parallel to each other.

FIGURES 7 and 8 show isometric and top views, respectively, of aphotoconductive unit in accordance with the present invention.

The element, indicated generally at 10, comprises a base 14 and themulti-layer photoconductive element 12. Contacting the top of theelement are electrical conducting means or terminals 16.

As shown in FIGURES 7 and 8, the electrical conducting means 16 needonly contact the upper or exposed surface of the element 1.2. Ifdesired, however, the electrical conducting means 16 may also contactthe sides of the element 12.

It is not necessary that the multi-layered element have a base orsupport. In FIGURE 9 there is shown an element 40 without a base. Thedotted lines 42 indicate the various layers. These layers, of course,cannot be seen, and the lines 42. are for purposes of illustration only.Conducting means 44 contact the top, sides and bottom of the element andend in electrodes 46.

As will be clear from the foregoing, the separately superimposed,separately heated in the presence of halogen, and separately cooledlayers of host photoconductive materials forming the devices disclosedherein are in intimate contact with one another. There is no dielectricor other separation between the layers Nor is there a visible interfacebetween the layers.

Although in any particular unit, each layer will ordinarily comprise thesame photoconductive host material or combination of materials, itshould be understood that for special purposes, the host material orcombination of host materials in one or more of the layers may bedifferent than that in the other layers.

Use of the multi-layer technique disclosed, in addition to altering thepeak response bandwidth and wavelength characteristics of thephotoconductive materials, additionally leads to an increase in thelight photocurrent of the materials, and more particularly to anincrease in the light photocurrent to dark photocurrent ratio, thisratio being referred to in the art as photosensitivity of the cell. Thischaracteristic will be made more clear from Example 3.

Example 3 Using the procedure and materials of Example 2, cadmiumsulfide Was applied in three layers, each 2.8 microns thick. Followingdeposition, heating in the presence of halogen and cooling to ambienttemperature of each layer,

the light and dark currents of the unit were measured.

Light current measurements were made at 2 foot-candle illuminations. Thepotential across the cell during measurement of :both light and darkphotocurrent was 60 volts/ D.C.

Following application of the first layer, the unit had a lightphotocurrent of 200 micro-amperes. In five seconds, the dark current was0.002 micro-ampere. Another layer was applied, heated in the presence ofhalogen and cooled. The composite bi layer device gave the followingphoto response: Light current was 400 micro-amperes at 2 footcandles. Infive seconds the dark current was .02 micro-ampere. A rise of 2:1 in thelight photocurrent and a 10:1 increase in the dark current was thusachieved. A third layer, of thickness identical with the other two, wasapplied, heated in the presence of halogen, and cooled to ambienttemperature, and measurements were made on the composite tri-layerelement. The light current increased to 800 micro-amperes at 2footcandles. In five seconds, the dark current had increased to only 0.2microampere.

In a further embodiment of the present invention, the peak responseWavelength of the multi-layer element may be further shifted, ifdesired. According to this embodiment, the photoconductive host materialor combination of materials in each layer, during heating in thepresence of halogen, or in the multi-layered element after completeformation thereof, is subjected to vapors of a material capable ofentering into an ion exchange reaction with the host photoconductivematerial present in the layers, and capable, as a result of suchreaction, or forming a photoconductive material with the host material.Typical of the vapors which may be employed with the photoconductivematerials disclosed herein may be mentioned sulfur, selenium, tellurium,and mixtures of the foregoing.

The temperature of 'contact between the vapors and the element willordinarily be between about and 1400" (2., preferably between about 5 00and 900 C.

The vapors of thematerials, such as selenium, tellurium, and sulfur, foruse in the ion exchange reaction, may be prepared by heating theseelements to a temperature at which the elements have a substantial vaporpressure, and then purging with an inert gas to cause entrainment ofvapors of the elements. The inert gas entrained with the elementalvapors can then be brought into Contact with each layer or with themulti-layered elements. Alternatively, the elements may be heated abovetheir sublimation temperature or boiling point to produce the requiredvapors, which may then be brought into contact with each layer duringheating in the presence of halogen or with the multi-layer unit as awhole.

Alternatively, the material capable of entering into a reaction with thehost material to transform the spectral response of the element to adifferent spectral response may be placed inside the furnace during anyof the heating steps disclosed herein. In this embodiment, theextraneous or non-host material need not be vaporized previous tocontact with the layers or the multi-layer element, and the vapors ofthe non-host or extraneous material will be generated in situ, so tospeak.

To cite an example of this embodiment, when the host material in'thelayers is cadmium sulfide, the peak response wavelength may be shiftedto the broad end of the spectrum by contacting each layer after orduring 'heating in the presence of halogen, or the entire element afterformation, with vapors of selenium, tellurium, and mixtures of theforegoing.

i As another example, when the host material is .cadrnium selenide,shifts in peak response wavelength and "I bandwidth are achievedbycontact, as described herein above, with vapors of sulfur, tellurium,or mixtures of the foregoing. As a further illustration of thisembodiment, when the host material is Cadmium telluride, the peakresponse bandwith and wavelength may be shifted by I contacting eachlayer after or during heating in the presence of halogen, or the overallelement after formation, with sulfur and/ or selenium vapors. i

FIGURES 3, 4 and 5 are curves illustrating the shift in peak responsewavelength when the host material in the multi-layered elements arecontacted with vapors of photosensitive materials different from thehost material. Curve S in each of these figures indicates schematicallythe visible light spectrum, the visible light color ranges being printedthereon.

In FIGURE 3, curve M, the host material is cadmium sulfide, and theelement has been subjected to'tellurium vapors or, stated differently,doped with tellurium. For curve N of FIGURE 3, the host material iscadmium telluride, and the element has been doped with sulfur.

In FIGURE 4, curve 0, the host material is cadmium sulfide, and theelement has been doped with selenium. In curve P of FIGURE 4, the hostmaterial is cadmium selenide, and the host material has been doped .withsulfur.

In FIGURE 5, curve Q, the host material is cadmium telluride, and thehost material has been doped with selenium vapors. In curve R of FIGURE5, the host material is cadmium selenide, and the host material has beendoped with tellurium.

In FIGURES 3, 4 and 5, the ordinates of the curves represent the time ofcontact of the photoconductive unit with the doping agent in minutes.The abscissae of the curves represent peak resposes in Angstrom units.The arrows on the curves indicate the directionin which the peakresponse is shifted, as a result of contact with the doping agent.

The following examples illustrate the embodiment of the presentinvention wherein the peak wavelength response may be further shifted,if desired, by using the doping technique described.

Example 4 Multi-layer units of cadmium sulfide were prepared followingthe procedure of Example 2 and following completion, were place in anoven at a temperature of about 500 to 700 C. j

In a separate oven was placed metallic selenium and the temperature wasraised to between about 500 and 700 C. I

The apparatus used in this example is shown schematically in FIGURE 10,in which 20 represents the furnace containing the multi-layered element22, and 24 represents the furnace containing the selenium 26. Thefurnaces are coupled by piping 30 provided with a heater 38, and furnace24 is provided with a gas inlet 32, while furnace 20 is provided with agas outlet 34.

An inert gas, such as nitrogen, preferably pre-heated to the temperatureof furnace 24, is fed to furnace 24 via inlet 32, sweeps over theselenium 26 and picks up selenium vapors. pors is fed into furnace 20containing multi-layered element 22 via piping 30. Preheater 38 preventscooling of the gases and precipitation of the selenium vapors in transitbetween the furnaces. The selenium vapors in the inert gas permeates themulti-layered element 22 in furnace 20 and causes a shift of thepeakresponse wavelength as indicated hereinabove.v

The process is continued until enough of the selenium vapor is picked upby the multi-layered element to shift the peak response Wavelength ofthe element the desired amount. Using this technique, peak responsewavelength of the multi-layered element having cadmium sulfide as thehost photoconductive material Was shifted in the direction of the arrowon curve 0 in FIGURE 4 from 5100 A. to 7300 A. by varying the time ofcontact of the multi- The inert gas enriched with selenium valayeredelements with the selenium vapors from 0 to about 2 minutes.

' Example 5 Example 4 was repeated, except that tellurium rather thanselenium was purged with nitrogen at a temperature of about 800 C.; andthe tellurium enriched nitrogen passed over the multi-layer element ofExample 2. The multi-layer element was held at a temperature of about500 to 300 C. Using this technique, peak response wavelengths ofmulti-layered elements having cadmium sulfide as the host material wereshifted in the direction of the arrow on curve M, FIGURE 3, from 5200 A.to 8500 A. by varying the time of contact of the multi-layered elementsWithtellurium vapors from 0 to about 5 minutes.

The use of photoconductive materials other than cadmium sulfide in thepresent invention will be clear from the following examples.

Example 6 Using the procedure of Example 2, a tri-layered element ofcadmium selenide polycrystalline material was prepared. Each of thelayers had a thickness of 2.8 microns. As a control, a tri-layered unitof cadmium selenide 8.4 microns thick was prepared following theprocedure of Example 1. The tri-layered unit had a peak response of 8725A. as compared with a peak response of the control of 7300 A.

' Example 7 Using the procedure of Example 2, a tri-layered element ofcadmium telluride polycrystalline material was prepared. Each of thelayers had a thickness of about 2.8 microns. As a control, a tri-layeredunit of cadmium telluride was prepared following the procedure ofExample 1. The tri-layered unit had a peak response of 8950 A. ascompared with a peak response of the control of 8500 A. V

' Example 8 Using the procedure of Example 4, multi-layered units ofcadmium selenide were subjected to sulphur vapors for varying periods oftime. Peak response wavelengths of the multi-layered elements havingcadmium selenide as the host material were shifted by this techniquefrom 7300 A. to 5200 A. in the direction of the arrow of curve P ofFIGURE 4 by varying the time of contact of the multi-layeredelementswith sulphur vapor from between about 0 to 1.0 minute.

Example 9 'Multi-layered elements of cadmium telluride were subjected tosulphur vapors using the procedure of Example 4 for Varying periods oftime. Using this procedure, peak response wavelengths of multi-layeredelements having cadmium telluride as the host material were shifted inthe direction of the arrow on curve N of FIGURE 3 from 8500 A. to 5200A. by varying the time of contact with sulphur vapors from between about0 and 5 minutes.

Example 10 Multi-layered elements of cadmium selenide were subjected totellurium vapors using the procedure of Example 4 for varying periods oftime. The peak response wavelengths of multi-layered elements havingcadmium selenide as the host material were shifted in the direction ofthe arrow on curve R of FIGURE 5 from 7300 A. to 8500 A. by varying thetime of contact with tellurium vapors from between about 0 and 5minutes.

Example 11 Multi-layered elements of cadmium telluride were subjected toselenium vapors using the procedure of Example 4. The peak responsewavelengths of multi-layered elements having cadmium telluride as thehost material were shifted in the direction of the arrow on curve Q ofFIG- URE 5 from 8500 A. to 7300 A. by varying the time of contact withselenium vapors from between about 0 and 5 minutes. 7

As has already been indicated, the ion exchange reaction between thehost material and the vapors of a different photoconductive material mayoccur following formation and heating of each layer if desired. In thisembodiment, contact between the host material and the vapors of thedoping material or materials may occur simultaneously during the heatingstep or may follow the heating step in the formation of each layer. Whenthe overall element is subjected to the doping materials, this mayconveniently be carried out simultaneously with heating of the lastdeposited layer. Of course, the ion exchange technique may also beemployed following complete formation of the element.

The following examples are illustrative of the embodiment whereinelemental mixtures of photoconductive materials are employed.

Example 12 Example 2 is repeated with the exception that a mixture ofelemental cadmium and elemental sulphur is substituted for cadmiumsulfide. The elements are present in the mixture in stoichiometricproportions, based on cadmium sulfide. Results comparable to those ofExample 2 were obtained.

Example 13 Example 6 is repeated with the exception that a mixture ofelemental cadmium and elemental selenium is substituted for cadmiumselenide. The elements are present in the mixture in stoichiometricproportions, based on cadmium selenide. Results similar to those ofExample 6 are obtained.

Example 14 Example 7 is repeated with the exception that a mixture ofelemental cadmium and elemental tellurium is substituted for cadmiumtelluride. The elements are present in the mixture in stoichiometricproportions, based on cadmium telluride. Results similar to those ofExample 7 are obtained.

Although in Examples 12 to 14, the elements are present in the mixturein stoichiometric proportions, it should be understood that any of theelements may be present in greater than or less than stoichiometricproportions. Thus, for example, the given equivalent weight ratio of oneelement to another element in the mixture of the embodiment underdiscussion may vary between about 0.1 and to 1, or even more.

Utilizing a stoichiometric excess of one element or another in themixture will lead to greater flexibility in shifting the peak spectralresponse of the multi-layered elements when the ion exchange techniquedescribed herein is employed as will be readily understood by thoseskilled in the art.

The invention in its broader aspects is not limited to the specificarticles and processes shown and described but departures may be madetherefrom within the scope of the accompanying claims without departingfrom the principles of the invention and without sacrificing its chiefadvantages.

What is claimed:

1. A photosensitive element comprising a host photoconductive material,said element having an electrical conductivity responsive to light andhaving a pre-determined peak spectral response wavelength and bandwidthdifferent from the peak spectral response wavelength and bandwidth ofthe host photoconductive material, said element comprising a pluralityof separately deposited, successively photo-sensitized layers of hostphotoconductive material, each of said layers being in the form of thinplatelets having a uniform thickness of between about 1.5 and 4.5microns and a surface area which is large in comparison to thethickness, the layers being substantially coterminous in surface area,and at least one surface of each of said layers being in immediate,intimate and substantially overall planar contact with a surface ofanother layer, the host photoconductive material in each layer beingphoto-sensitized by a halogen, the host photoconductive material in eachlayer being a combination of at least two substances, one of saidsubstances being selected from the group consisting of cadmium, zinc,mercury and mixtures thereof, the other of said substances beingselected from the group consisting of sulfur, tellurium, selenium andmixtures thereof.

2. The photosensitive element of claim 1 including electrical conductingmeans in contact with the element.

3. A photosensitive element according to claim 1, wherein the hostphotoconductive material in each layer is in polycrystalline form.

4. A photosensitive element according to claim 11, wherein thesubstances in said combination are present in stoichiometricproportions.

5. A photosensitive element according to claim 1, wherein saidcombination of substances is a mixture thereof in elemental form. r

6. A photosensitive element according to claim 5, wherein the gramequivalent ratio of either one of said substances to the other isbetween 0.1 to 1 and 10 to 1.

7. A photosensitive element according to claim 1, wherein saidcombination of substances is a metal salt selected from the groupconsisting of cadmium sulphide, cadmium selenide, cadmium telluride,zinc sulphide, zinc selenide, Zinc telluride, mercury sulphide, mercuryselenide, mercury telluride, and mixtures of at least two thereof.

8. A photosensitive element according to claim 1, wherein said pluralityof layers may be any number ranging from 2 to at least 10.

9. A photosensitive element according to claim 1, wherein thecombinaations of substances in a least two of the layers are the same.

10. A photosensitive element according to claim 1, wherein thecombinations of substances in at least two of the layers are different.

References Cited by the Examiner UNITED STATES PATENTS 2,732,469 1/56Palmer 250-211 X 2,742,550 4/56 Jenness 250-211 X 2,900,523 8/59 Ruzicka250-211 2,961,542 11/60 Cartwright et al. 250-83 3,005,107 10/61Weinstein 250-211 3,076,949 2/63 Anderson 250-211 X RALPH G. NILSON,Primary Examiner.

ARCHIE R. BORCHELT, FREDERICK M. STRADER,

Examiners,

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No 3,191,045 June 22 1965 Robert Colman It is hereby certified that errorappears in the above numbered patent reqliring correction and that thesaid Letters Patent should read as correctedbelow.

In the heading to the drawings, Sheets 1 to 4, line 1, fc "R. M.WILMOTTE", each occurrence, read R. COLMAN Signed and sealed this 24thday of August 1965.

(SEAL) Altest:

RNEST W. SWIDER EDWARD J. BRENNER Nasting Officer Commissioner ofPatents

1. A PHOTOSENSITIVE ELEMENT COMPRISING A HOST PHOTOCONDUCTIVE MATERIAL,SAID ELEMENT HAVING AN ELECTRICAL CONDUCTIVITY RESPONSIVE TO LIGHT ANDHAVING A PRE-DETERMINED PEAK SPECTRAL RESPONSE WAVELENGTH AND BANDWIDTHDIFFERENT FROM THE PEAK SPECTRAL RESPONSE WAVELENGTH AND BANDWIDTH OFTHE HOST PHOTOCONDUCTIVE MATERIAL, SAID ELEMENT COMPRISING A PLURALITYOF SEPARATELY DEPOSITED, SUCCESSIVELY PHOTO-SENSITIZED LAYERS OF HOSTPHOTOCONDUCTIVE MATERIAL , EACH OF SAID LAYERS BEING IN THE FORM OF THINPLATELETS HAVING A UNIFORM THICKNESS OF BETWEEN ABOUT 1.5 AND 4.5MICRONS AND A SURFACE AREA WHICH IS LARGE IN COMPARISON TO THETHICKNESS, THE LAYERS BEING SUBSTANTIALLY COTERMINOUS IN SURFACE AREA,AND AT LEAST ONE SURFACE OF EACH OF SAID LAYERS BEING IN IMMEDIATE,INTIMATE AND SUBSTANTIALLY OVERALL PLANAR CONTACT WITH A SURFACE OFANOTHER LAYER, THE HOST PHOTOCONDUCTIVE MATERIAL IN EACH LAYER BEINGPHOTO-SENSITIZED BY A HALOGEN. THE HOST PHOTOCONDUCTIVE MATERIAL IN EACHLAYER BEING A COMBINATION OF AT LEAST TWO SUBSTANCES, ONE OF SAIDSUBSTANCES BEING SELECTED FROM THE GROUP CONSISTING OF CADMIUM, ZINC,MERCURY AND MIXTURES THEREOF, THE OTHER OF SAID SUBSTANCES BEINGSELECTED FROM THE GROUP CONSISTING OF SULFUR, TELLURIUM, SELENIUM ANDMIXTURES THEREOF.